The present invention relates to a metal powder coated with an organic dye and a polymeric shaped article impregnated or filled with such a dye-coated metal powder or, in particular, a so-called plastic magnet impregnated with a dye-coated metallic magnet powder.
In recent years, wide applications are found by a variety of polymeric compositions impregnated or filled with a fine metallic powder utilizing their unique electric and magnetic properties along with the development of the electronic technology. Such polymeric compositions including coating compositions, e.g. paints, rubbers and plastics have both of the advantages of the polymeric materials, e.g. moldability and flexibility, and the metallic components, e.g. electroconductivity and magnetism.
The finely divided metallic dispersant used in these composite materials are formed of fine particles, fibrils, flakes or chips and these powders have an extremely large surface area per unit volume. A problem accompanying the extremely large surface area and the high surface activity of the metallic powder is the high susceptibility thereof to oxidation by the atmospheric oxygen which adversely affects the performance of the composite materials filled with such a surface-oxidized metallic powder. For example, an excessively high oxidizability of the metal powder sometimes causes fire by the spontaneous oxidation in the course of the fabrication of the polymer-based composition at an elevated temperature to decrease the yield of the acceptable products. In addition, the oxidation of the metal powder is responsible to the shortened serviceable life of the products filled with the metal powder in the long-run use.
In connection with so-called plastic magnets, i.e. permanent magnets of a magnetic powder dispersed in a matrix of or shaped with a plastic resin, as a type of the above mentioned metal powder-filled polymeric composite materials, plastic magnets have several advantages over other types of permanent magnets prepared by the powder metallurgical process or by casting of a molten magnetic metal or alloy which suffer from the disadvantages of the extremely high costs for the preparation of a magnet of a complicated configuration with high precision of the dimensions, difficulties in obtaining a uniform distribution of the magnetism in the magnet, difficulties in the integration of the magnet with other parts and difficulties in the preparation of a radially anisotropic or multipolar anisotropic magnet due to cracking. Plastic magnets are outstandingly advantageous in these respects.
As is well known, the magnetic powder used in plastic magnets at the early stage of the development thereof has been the magnetic ferrites due to the inexpensiveness and stability against oxidation. In recent years, however, more powerful plastic magnets are required with reduction in size and weight so that the ferrite powders are being replaced with metallic permanent magnets with higher magnetic performance such as the rare earth-cobalt based permanent magnets.
While the powder of such a rare earth-cobalt magnet as a magnet powder in plastic magnets is unquestionably excellent in the magnetic performance over the conventional ferrites, a problem in the use of a metallic magnet powder as a magnet powder in plastic magnets is the oxidation by the atmospheric oxygen so that rare earth-cobalt based magnet powders rapidly lose their magnetic properties when heated in a high temperature atmosphere of air at 200.degree. to 250.degree. C. or higher in addition to the danger of ignition in the course of the fabrication of the composition.
Several methods have been proposed, of course, for the prevention of surface oxidation of fine metallic powders including (1) the method of ion plating and (2) the method of coating with a synthetic resin for the surface treatment. The method of ion plating is, however, economically disadvantageous because the method is performed by use of a specific apparatus with a large consumption of energy along with the very low productivity. The method of resin coating is, on the other hand, not free from the problems that the electric conductivity of the composition filled with such a resin-coated metal powder is necessarily remarkably decreased and the coating film of the resin is readily destroyed resulting in the loss of protection against air oxidation.
Limiting the subject matter to the plastic magnets filled with a powder of rare earth-cobalt magnets, several measures have been proposed to get rid of the problem of the air oxidation of the magnet powder as follows.
(1) Blending of the magnet powder with a plastic resin is performed in an atmosphere of an inert gas. This method is indeed effective to some extent to prevent air oxidation of the magnet powder although the effect can never be complete and the method necessarily leads to the decrease in the productivity and increase in the production cost.
(2) The magnet powder is treated in advance for surface coating with a titanium-containing or silane-based surface treatment agent. This method is also effective to some extent but the effect can never be complete and hardly be expected at 300.degree. C. or higher.
(3) Plastic magnets are prepared by use of a plastic resin having a sufficient flowability at a relatively low temperature so that the shaping of the plastic magnet by molding can be performed at a correspondingly low temperature consequently with little oxidation of the magnet powder. A problem in this method is, as a matter of course, the correspondingly low upper limit of the usable temperature range of the plastic magnet and, in addition, such a plastic magnet exhibits a relatively rapid decrease in the magnetic properties in the course of long-run use.
(4) The plastic magnet is prepared with a decreased loading amount of the magnet powder in the matrix of the plastic resin. Such a method is, however, applicable only when no high performance plastic magnet is desired.
Thus, no effective and convenient method is known in the prior art for preventing surface oxidation of a metal powder as a dispersant in a polymer-based composite material or, in particular, preventing surface oxidation of a powder of a metallic magnet as a magnetic dispersant in a plastic magnet.